Plastics Technology

AUG 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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weak retainer plate from separating from the ejector plate as the assembly retracts. The formula for calculating the amount of deflection a rectangular plate will have under a specific set of condi- tions states that the plate's resistance to deflection is proportional to the third power of its thickness. In that same formula, the unsupported length is also proportional to the third power. This is why it is so important for the machine's ejector bars to be evenly spaced. This deflection formula is important when molding a part with a large amount of projected area or calculating how thick to make a support plate or a cavity plate in a hot-runner mold. But when it comes to an ejector plate, the standard 1-in. or 1 1/8-in. thickness is almost always sufficient. If you have a plate-deflection issue, the root cause of your problem is probably not the thickness of the plate; more likely it's an uneven loading issue. However, I do recall a large mold or two where we used a 1 3/8-in.-thick ejector plate, but only because it was riddled with holes from support pillars and numerous other mold components. Once I went so far as to heat treat an ejector plate made of S-7 steel and guided it with linear ball bearings for an elec- trical connector molded in liquid-crystal polymer. The reason wasn't possible plate deflection. The concern was that the ejector-pin height tolerance on the drawing was +0.0000 to -0.0005 in. If the head of the pin made even a slight depres- sion into the face of a softer ejector plate, or if the ejector plate was even slightly out of alignment, the parts would be rejected. While it is a little beyond the scope of this article, I should point out that it is good practice for ejector pins to be 0.0005 in. to 0.0015 in. below the surface of the part they are ejecting. This helps ensure the part will not hang up on the pin, which would require additional ejector pulses. It also helps reduce stress marks and "skid" marks on the part itself. Conversely, if the ejector pins are slightly into the part, you can get aesthetic issues caused by turbulent material flow, particularly on thin-walled parts. You may have seen this happen on a molded part with depressed engraving, formed by raised engraving in the steel. Thermal expansion is a major cause of ejector-pin misalignment, especially on large molds that run at elevated temperatures. Just because the ejector plates move freely back and forth on the bench doesn't mean they won't bind up under production Ejector plates should travel parallel to the centerline of the mold, and receive and apply an evenly distributed load. Which ElboW ProblEm Do You NEED to SolvE? ProblEm cAuSE SolutioN Formation of streamers, angel hair and snake skins when conveying plastic pellets Plastic pellets skidding and/or bouncing against the outside radius of sweep elbows create friction and heat, melting pellet surfaces. Smart Elbow ® replacements from hammertek eliminate these problems by eliminating impact Unlike conventional "impact" elbows and "plugged-tee" elbows that rely on material impact to change direction, HammerTek's Smart Elbow ® design features a spherical chamber that protrudes partially beyond the desired 90º or 45º pathway, which causes a ball of pellets suspended in air to rotate, gently deflecting incoming pellets around the bend. No impact means no wear and no fines and no streamers, angel hair or snake skins. Wear and failure when conveying glass-filled, mineral-filled or other abrasive pellets Abrasive pellets hit the outside radius of conventional impact elbows at high speed, continually wearing through the elbow wall. FF-0975 Free, No-risk trial offer See how Smart Elbow® deflection elbows eliminate conventional and plugged-tee impact elbow problems—at no cost or obligation. 1-610-814-2273 45° and 90° elbows available in diameters ranging from 1.25 to 18 inch @plastechmag 29 Plastics Technology T O O L I N G

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